Phenylpropanoids classes

Flavonoids are the largest class of phenylpropanoids in plants. The basic flavonoid structure is two aromatic rings (one from phenylalanine and the other from the condensation of three malonic acids) linked by three carbons (Fig. 3.6). Chalcone is converted to naringenin by the enzyme chalcone isomerase, which is a key enzyme in flavonoid synthesis. This enzyme, like PAL and chalcone synthase (CHS), is under precise control and is inducible by both internal and external signals. Naringenin is the... 

Neolignans are also compounds composed of two phenylpropanoid units, but linked in a manner other than C8-C8 (Fig. 12.1) [10]. The compounds of this class are often chiral and naturally occurring neolignans are usually optically active, which evokes the involvement of DPs in the biosynthesis of neolignans. 

Using this system, (Z)-hinokiresinol isolated from cultured cells of A. officinalis was determined to be the optically pure (75 )-isomer, while ( )-hinokiresinol isolated from cultured cells of C. japonica had 83.3% e.e. in favor of the (7S)-enantiomer (Table 12.1). The enzymatically formed (Z)-hinokiresinol obtained following incubation of p-coumaryl p-coumarate with a mixture of equal amounts of recZHRSa and recZHRSf) was found to be the optically pure (75)-isomer, which is identical to that isolated from A. officinalis cells (Table 12.1). A similar result was obtained with the crude plant protein from A. officinalis cultured cells, where the formed (Z)-hinokiresinol was almost optically pure, 97.2% e.e. in favor of the (75)-isomer (Table 12.1). In sharp contrast, when each subunit protein, recZHRSa or recZHRSP, was individually incubated with p-coumaryl p-coumarate, ( )-hinokiresinol was formed (Table 12.1). The enantiomeric compositions of ( )-hinokiresinol thus formed were 20.6% e.e. (with recZHRSa) and 9.0% e.e. (with recZHRSP) in favor of the (7S)-enantiomer (Table 12.1). Taken together, these results clearly indicate that the subunit composition of ZHRS controls not only cis/trans selectivity but also enantioselectivity in hinokiresinol formation (Fig. 12.3). This provides a novel example of enantiomeric control in the biosynthesis of natural products. Although the mechanism for the cis/trans selective and enantioselective reaction remains to be elucidated, for example by x-ray crystallography, the enantioselective mechanism totally differs from the enantioselectivity in biosynthesis of lignans, another class of phenylpropanoid compounds closely related to norlignans in terms of structure and biosynthesis. 

Volatile compounds are often involved in long distance attraction and are especially important as attractants and repellents (as defined by Kogan, ). One major class of volatile materials, essential oils, is comprised of complex mixtures of terpenes, phenylpropanoid derived compounds and a number of esters, alcohols, aldehydes, ketones, acids, and hydrocarbons. The constituent compounds are mostly of low to medium molecular weight and generally not highly oxygenated. Some of the biological properties of these compounds have been reviewed (17,41,46,55,56). 

Studies of other Polygonum species are under way to establish the presence of homologs of the vanicosides. Isolation and structure elucidation of these homologs, along with the preparation of derivatives by synthetic methodology, will facilitate the establishment of a library of compounds which will be screened for PKC inhibitory activity. The results will produce a better understanding of the structure-activity relationships inherent in this class of phenylpropanoid glycosides. 

The phenolics include anthocyanins, anthraquinones, benzofurans, chromones, chromenes, coumarins, flavonoids, isoflavonoids, lignans, phenolic acids, phenylpropanoids, quinones, stilbenes and xanthones. Some phenolics can be very complex in structure through additional substitution or polymerization of simpler entities. Thus xanthones can be prenylated and flavonoids, lignans and other phenolics can be glycosylated. Condensed tannins involve the polymerization of procyaninidin or prodelphinidin monomers and hydrolysable tannins involve gallic acid residues esterified with monosaccharides. As detailed in this review, representatives of some major classes of plant-derived phenolics are potent protein kinase inhibitors. 

Fig. (1). Schematic view of some branches of phenylpropanoid metabolism. Solid arrows indicate enzymatic reactions with the respective enzyme indicated on the right. PAL, phenylalanine ammonia-lyase C4H, cinnamate 4-hydroxylase 4CL, 4-coumarate CoA ligase CHS, chalcone synthase CF1, chalcone flavavone isomerase F3H, flavanone 3-hydroxylase DFR, dihydroflavonol reductase CHR, chalcone reductase. Broken arrows indicate metabolic branches towards several classes of phenylpropanoids, or several subsequent enzymatic steps. In some cases the enzymes indicated are also involved in other reactions, not shown.

In plants, biosynthesis and exudation of allelochemicals follows developmental, diurnal, and abiotic/biotic stress-dependent dynamics. Compounds from 14 different chemical classes have been linked to allelopathic interactions, including several simple phenolic acids (e.g., benzoic and hydroxycinnamic acids) and flavonoids [Rice, 1984 Macias et al., 2007]. The existence of several soil biophysical processes that can reduce the effective concentration and bioactivity of these compounds casts doubts on their actual relevance in allelopathic interactions [Olofsdotter et al., 2002]. However, there are well-documented examples of phenylpropanoid-mediated incompatible interactions among plants. Several Gramineae mediate allelopathic interactions by means of... 

Methylation is one of the most common enzymatic modifications in plant specialized (secondary) metabolism. Almost all classes of plant specialized metabolites are known to be methylated, including amino acids, alkaloids, phenylpropanoids, sugars, purines, sterols, thiols, and flavonoids. The methyl transfer most commonly occurs on C, N, S, or O atoms. 

MAURY, S., GEOFFROY, P., LEGRAND, M., Tobacco O-methyltransferases involved in phenylpropanoid metabolism. The different caffeoyl-coenzyme A/5-hydroxyferuloyl-coenzyme A 3/5-O-methy (transferase and caffeic acid/5-hydroxyferulic acid 3/5-O-methyltransferase classes have distinct substrate specificities and expression patterns., Plant Physiol., 1999,121,215-223. 

Phenylpropanoids have an aromatic ring with a three-carbon substituent. Caffeic acid (308) and eugenol (309) are known examples of this class of compounds. Phenylpropanoids are formed via the shikimic acid biosynthetic pathway via phenylalanine or tyrosine with cinnamic acid as an important intermediate. Phenylpropanoids are a diverse group of secondary plant compounds and include the flavonoids (plant-derived dyes), lignin, coumarins, and many small phenolic molecules. They are known to act as feeding deterrents, contributing bitter or astringent properties to plants such as lemons and tea. 

Aroma-active molecules of natural origin are mainly formed via well-known biosynthetic pathways.17,27-29 The major class is the terpenoids followed by phenylpropanoid compounds (see Chapters 1.15, 1.16, and 1.24). Enzymatic and biosynthetic transformation and cleavage of fatty acid is another important source of aroma-active compounds (see Chapter 8.07). Transformation of amino acids and carbohydrates by fermentation is also... 

The synthesis of /raw5-resveratrol oligomers requires the set of enzymes that yield trows-resveratrol from the phenylpropanoid precursor, p-coumaryl-CoA and malonyl-CoA, and then the oxidative coupling of resveratrol units by a phenol oxidase, which is presumably a class HI plant peroxidase [138,139], In fact, class HI plant peroxidases, such as that purified from grapevines, are able to oxidize frvms-resveratrol with values of 17-93 pM for H2O2 concentrations ranging from 0.1 to 5.0 mM at pH 4.0 [58], and with values of 11.9 pM" s, at the same pH [35,58]. 

Figure 4 Exemplar structures of various antibiotic classes that bind to either the 505 or the 305 subunit. Macrolides azithromycin (1), oxazolidinones linezolid (2), aminoglycosides Kanamycin A (3), Pleuromutilin (4), phenylpropanoids chloramphenicol (5), lincosamides clindamycin (6), Sparsomycin (7), Anisomycin (8), and tetracycline (9). See Scheme 9 for thiosptrepton (38). Not pictured streptogramins such as quinupristin/dalfopristin.

Flavonoids are one of the largest classes of phenylpropanoid-derived plant specialized metabolites, with 10,000 different members. They consist of two main groups, the 2-phenylchromans (flavonoids flavanones, flavones, flavonols, flavan-3-ols, anthocy-anidins) and the 3-phenylchromans (isoflavonoids isoflavones, iso-flavans, pterocarpans). Some flavonoids and their metabolites exhibit positive effects for disease therapy and chemoprevention [218,219],... 

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